1. Field of the Invention
This invention relates to a process for producing a vinyl chloride polymer by suspension polymerization of vinyl chloride monomers or a monomer mixture containing them.
2. Description of the Prior Art
The suspension polymerization of vinyl chloride monomers or a monomer mixture thereof in an aqueous medium are usually carried out in a polymerization vessel provided with a cooling/heating jacket, a stirrer, a material charging line, a baffle, a polymer draw-out line, an unreacted monomer recovery line and so forth. To polymerize the vinyl chloride monomers or monomer mixture in an aqueous medium in such a polymerization vessel, a process is employed in which the aqueous medium, a dispersant, the vinyl chloride monomers or monomer mixture, a polymerization initiator and optionally other additives are fed through the material charging line, provided at the gas-phase upper part of the polymerization vessel, into the polymerization vessel with stirring and thereafter these are heated to carry out polymerization.
In this polymerization process, polymer scales tend to adhere to the polymerization vessel inner wall, stirrers and so forth. Such polymer scales bring about problems that those having adhered thereto may cause a decrease in cooling capacity of the polymerization vessel, and also those having come off may mix in products, resulting in a low product quality. Much labor and time must be taken to remove such polymer scales having adhered. Also, since the polymer scales contain unreacted monomers, there is a problem of environmental hygiene.
Accordingly, in order to prevent polymer scales from adhering, a polymerization vessel is conventionally used whose inner wall and so forth have been coated with a polymer scale preventive agent to form its coating film.
Use of such a polymerization vessel, however, is still disadvantageous in that, with repetition of polymerization, the polymer scales tend to adhere to the polymerization vessel inner wall and especially to the baffle provided in the polymerization vessel, on its side facing the polymerization vessel inner wall at the interface between gas phase and liquid phase (hereinafter "gas/liquid interface") and the vicinity thereof.
Once polymer scales have adhered to the baffle on its side facing the polymerization vessel inner wall at the gas/liquid interface and the vicinity thereof, the polymer scales grow gradually with repetition of polymerization, and the polymer scales having grown may come off and mix in the resulting polymer to cause fish eyes in a large number, resulting in a poor product quality. Such a problem has been involved.
In recent years, for the purpose of improving productivity, an oil-soluble polymerization initiator has been used whose temperature at which the half-life comes to be 10 hours in the state of a 0.1 mol/L benzene solution is 30 to 55.degree. C., in order to shorten polymerization time (Japanese Laying-open Publication (kokai) Nos. 8-109203 and 8-225613). However, there is another problem that such an oil-soluble polymerization initiator is active at relatively low-temperature and hence the polymerization carried out using the oil-soluble polymerization initiator tends to result in a poor product quality, e.g., to cause fish eyes in a large number in the resulting polymer.